1. Field of the Invention
The present invention relates to an electric power storage lithium-ion secondary battery, and more particularly to a lithium-ion secondary battery using a sintered negative electrode and a manufacturing method thereof.
2. Description of Related Art
Secondary batteries are nowadays widely used as power sources for cellular phones, notebook computers, digital cameras, and compact video cameras, for example. Among various types of secondary batteries, lithium-ion secondary batteries have been increasingly widespread that use, for example, lithium cobaltate (LiCoO2), lithium nickelate (LiNiO2), Li (Co1-XNiX)O2 that is a solid solution thereof, or LiMn2O4 having a spinel structure as a positive electrode material, and use a carbon material such as graphite as a negative electrode material, and that use an electrolyte which is a lithium compound (a solute) in a liquid organic compound (a solvent).
When a lithium-ion secondary battery charges, a lithium atom (Li) in a lithium transition metal oxide, which is a positive electrode active material, turns into a lithium ion (Li+) and then slides in between the carbon layers of the negative electrode (intercalation), and, when the lithium-ion secondary battery discharges, the lithium ion (Li+) slides out between the carbon layers (deintercalation), then moves toward the positive electrode, and then turns back to the lithium compound. In this way, a charge/discharge reaction proceeds. In addition to such advantages as having a higher output voltage and a higher energy density than a nickel-cadmium battery and a nickel hydride battery, the advantage of this lithium-ion secondary battery is that it has no “memory effect”, which is an effect associated with a cycle of a shallow discharge and a subsequent recharge, resulting in a decrease in the apparent discharge capacity.
In such a lithium-ion secondary battery, electrodes are formed by applying an active material on a metal leaf, and then the battery is formed by stacking or winding up the electrodes thus formed. Conventionally, the lithium-ion secondary battery is mainly used as a power source for a portable device. In recent years, however, attempts have been made to achieve a larger lithium-ion secondary battery having a capacity of 5 Ah or more for electric power storage purposes.
The lithium-ion secondary battery is known not only for its high output voltage and high energy density as described above, but also known as having a high energy efficiency (discharge electric power/charge electric power), and these properties are desirable for an electric power storage battery. However, the lithium-ion secondary battery does not satisfactorily meet users' demands in terms of the number of charge and discharge cycles, that is, a cycle life, and therefore needs to be improved. A longer cycle life of the lithium-ion secondary battery is particularly required when it is used for energy storage purposes or used as an electric power source for an electric automobile.
On the other hand, when adhering to a conventional method of manufacturing the above-described small battery, electrodes are formed by applying an active material onto a metal leaf and attaching it there by pressurizing, and then the electrodes thus formed are wound up or stacked. However, manufacturing a large battery by this method would require more complicated manufacturing procedures than a small battery when winding up or stacking the electrodes, because the large battery has a greater capacity and a wider electrode area than the small one, and thus inconveniently leads to a greatly reduced manufacturing efficiency.
Specifically, in a currently commercially available small lithium-ion secondary battery having a capacity of 3 Ah or less, the capacity per unit electrode area offered by an active material thereof is between 2 mAh/cm2 and 3 mAh/cm2 to achieve satisfactory load characteristics. If this small lithium-ion secondary battery is used to form a large battery having a capacity of 5 Ah or more, the required electrode area would be between 1600 cm2 and 2500 cm2. The necessity of such an electrode with a very wide area requires the electrodes to be stacked or wound up, and furthermore it is necessary to stack a large number of electrodes or wind up the electrodes an increased number of times. On top of that, allowing a battery to have a capacity of 5 Ah or more increases the required electrode area, making it difficult to manufacture an electrode itself.
In the present specification, the “capacity per unit area” of an electrode and the “weight of the active material per unit area” of the electrode are defined as follows. On the face of a positive or negative electrode facing an electrode of the opposite polarity, suppose a 1 cm2 square and then suppose the volume of a rectangular parallelepiped having that square as its bottom side and having a height equal to the thickness of the electrode. Then the capacity of the electrode and the weight of the active material contained in the electrode as measured per such unit volume are referred to as the “capacity per unit area” of the electrode and the “weight of the active material per unit area” thereof respectively. Note that the unit of the “capacity per unit area” is “mAh/cm2”.
On the other hand, the mere increase in size of a lithium-ion secondary battery raises the level of energy to be stored in the lithium-ion secondary battery, resulting in, for example, a high current flow when the positive and negative electrodes are short-circuited together. This indicates that there are not enough safety measures in place to deal with abnormal events.
A method is disclosed in JP-A-H11-322314 (hereinafter referred to as Patent Publication 1) that prevents a short circuit at the time of forming a battery by forming a negative electrode for a nonaqueous secondary battery by applying slurry onto a copper foil—the slurry prepared by mixing together carbon fiber powder as a negative electrode active material, acetylene black as an electrically conductive member, and polyvinylidene fluoride as a binder, then adding N-methylpyrrolidone thereto, and then kneading them together—and then drying it. Moreover, a technique is disclosed in JP-A-2001-40548 (hereinafter referred to as Patent Publication 2) that achieves a high-capacity secondary battery by using carbon fiber obtained by sintering cellulose fiber together with calcium carbonate containing magnesium carbonate at 800° C. or more as a material of an electrode for an electric double layer capacitor.
However, Patent Publications 1 and 2 do not describe how to improve a cycle life of a lithium-ion secondary battery, and therefore it is not possible to address the above-described challenges of realizing a lithium-ion secondary battery having a longer cycle life as well as a higher-capacity.